It is well known that a mobile instrument can operate with power supplied from a battery. For example, battery voltage is regulated to constant level by a voltage regulator and is supplied to an interior circuit. In order to detect a low voltage of the battery, a low voltage detection circuit is independently provided to monitor battery voltage in order to prevent the voltage regulator and interior circuit from erroneous operations as discussed in Japanese Patent Application Laid Open No. 2001-69677.
FIG. 3 illustrates an exemplary circuit of a background power supply circuit. As shown, the power supply circuit is built in a background mobile instrument and receives power supply from a battery 1. The power supply circuit supplies an output voltage made constant by a voltage regulator, formed from a reference voltage source 2, a plurality of MOS transistors 3 to 8, and a pair of feedback resistances 9 and 10, through a MOS transistor 5 serving as an output driver to the interior circuit 12 as a regulator output.
Among the plurality of MOS transistors 3, 4, 5, 6, 7, and 8 transistors 34, 5 are P type MOS transistors, while transistors 6, 7, 8 are N-type MOS transistors.
Further, in order to avoid an erroneous operation in the interior circuit 12, a low voltage detection circuit 11 detects whether a battery voltage has decreased to a prescribed level, and outputs a control signal (e.g. a system reset) to the interior circuit 12. Thus, in order to compare voltages of the reference voltage source 2 and battery 1, the low voltage detection circuit 11 includes an amplifier for comparison use employing a MOS transistor, for example, as in the above-mentioned voltage regulator.
Further, in the power supply circuit of FIG. 3, when the battery 1 decreases a voltage, the voltage regulator stops operating and comes to an inert condition incapable of outputting a constant voltage. However, even in the inert condition, the voltage regulator continues to consume current.
FIG. 4 illustrates an exemplary circuit of another background power supply circuit. In the power supply circuit of FIG. 4, when the low voltage detection circuit 11 detects that the battery voltage has decreased to a prescribed level, the reference voltage source 2 and the voltage regulator are stopped via inverter 14 and MOS transistors 13 and 15 to suppress current waste.
However, in the power supply circuit illustrated in FIGS. 3 and 4, despite that comparison is necessarily performed with a voltage of the reference voltage source 2, the low voltage detection circuit 11 and voltage regulator are separated, resulting in a large number of circuit parts and increasing the cost of the power supply circuit. In addition, the large number of circuit parts unnecessarily increases power consumption.